I think you need to talk to a solicitor or Citizens Advice Bureau to ensure you get the correct advice. You may also ask a builder to take a look to check that you are right to blame this rockery although it does seem likely. If you build an extension against another property you need to get a Party Wall Agreement in place first so I doubt what they have done is legal

If you type 'Garden Design Software' it comes up with loads some aimed at the professionals and others at amateurs. There is a free download list from Gardeners World shown. Anyway it might be worth you looking.